The present invention relates generally to integrated circuits, and more specifically, to integrated circuits containing active circuitry and fixed capacitors.
Voltage tuneable capacitors (varactors) and fixed capacitors are conventionally used together in a wide variety of different electrical circuits. Such circuits include, for example, tuneable circuits (e.g., lowpass, highpass, bandpass, or bandstop filters) or voltage controlled oscillators (VCO's). In such circuits, the varactor is used in conjunction with a variable applied voltage, and one or more fixed capacitors, to "tune" the output of the circuit. In the case of filter circuits, the voltage applied to the varactor is varied to "tune" the output voltage response of the filter as a function of the frequency of the input signal. An example of a conventional varactor-tuned filter is disclosed in U.S. Pat. No. 5,107,233. In the case of a VCO, the voltage applied to the varactor is varied to "tune" the output frequency of the oscillator. An example of a conventional varactor-controlled oscillator is disclosed in U.S. Pat. No. 5,694,092.
Tuneable filter circuits and VCO's have applicability in a wide variety of electrical devices including devices that require the use of integrated circuitry due to constraints on acceptable circuit dimensions. Such constraints are common in circuitry contained in many commercially available devices, including for example, transceiver circuitry contained in mobile radio telephones. The use of integrated circuits in such devices permits the circuitry to be disposed within smaller housings thus allowing for easier portability of the overall device. Tuneable filter circuits and VCO's are therefore commonly fabricated as semiconductor integrated circuits in many devices. An example of a monolithic integrated VCO is disclosed in U.S. Pat. No. 4,458,215 to Huang. As shown in FIGS. 1 and 2 of this patent, the varactors (52, 54, 56, 68) and fixed capacitors (15, 16) are typically fabricated on a specified area of a semiconductor substrate (20). Drawbacks with this conventionally configured integrated circuit include, however, a relatively large surface area due to the side by side disposition of the varactors (52, 54, 56, 68) and capacitors (15, 16) and the parasitic capacitance associated with the relatively lengthy interconnections used between the VCO components. The total cost of an integrated circuit, which includes a circuit such as the VCO disclosed in Huang, is a function of the amount of semiconductor area consumed by the fabricated circuitry. Furthermore, the distance between components disposed side by side in an integrated circuit increases the parasitic capacitance associated with the interconnections between components. This parasitic capacitance can reduce the tuneable range of the VCO and otherwise be detrimental to VCO performance. The side by side disposition of the varactors and fixed capacitors of conventional integrated circuits such as Huang therefore increases the parasitic capacitance associated with the integrated circuit and the relative cost associated with constructing the integrated circuit.